Folly failed to build due to error C2027 on MSVC

Hi All,

Folly failed to build due to error C2027 on MSVC. This issue can be reproduced on latest version. Could you please help take a look at this? Thanks in advance!

**Environment**
VCPKG latest revision + Folly latest version + VS 2019 + Windows Server 2016

**Repro steps:**

1. open X64 native tools command prompt for VS2019
2. git clone https://github.com/Microsoft/vcpkg D:\VCPKG\src
3. cd d:\vcpkg\src
4. bootstrap-vcpkg.bat
5. set path=%cd%;%path%
6. vcpkg install folly:x64-windows -–head

reference: https://developercommunity.visualstudio.com/content/problem/525532/visual-studio-2019-compilation-issue.html

**Actual result:**
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2027: use of undefined type 'std::basic_ostream>'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\iosfwd(217): note: see declaration of 'std::basic_ostream>'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(4706): note: see reference to function template instantiation 'std::basic_ostream> &std::_Insert_string,unsigned __int64>(std::basic_ostream> &,const _Elem *const ,const _SizeT)' being compiled
with
[
_Elem=char,
_SizeT=unsigned __int64
]
F:\gitP\microsoft\vcpkg\buildtrees\folly\src\head\master-d012b8a853\folly/experimental/symbolizer/SymbolizedFrame.h(80): note: see reference to function template instantiation 'std::basic_ostream> &std::operator <<,std::allocator>(std::basic_ostream> &,const std::basic_string,std::allocator> &)' being compiled
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2065: 'iostate': undeclared identifier
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2146: syntax error: missing ';' before identifier '_State'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2065: '_State': undeclared identifier
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2027: use of undefined type 'std::basic_ostream>'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\iosfwd(217): note: see declaration of 'std::basic_ostream>'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.26.28801\include\xstring(482): error C2065: 'goodbit': undeclared identifier